1. Field of the Invention
The present invention relates to an electrostatic latent image developing agent used in electrophotography, electrostatic recording, and electrostatic printing. The invention relates also to an image forming method using the electrostatic latent image developing agent.
2. Description of the Related Art
Recently, a method for visualizing image information by utilizing an electrostatic latent image, such as electrophotography, has become popular in various fields. Conventionally, in electrophotography, a latent image is formed on a photoreceptor or an electrostatic recording medium and charge detecting particles called toner are adhered to the electrostatic latent image to develop and visualize the latent image.
Electrostatic latent image developing agents (hereinafter also referred to as “developing agents”) are classified, in a broad sense, into a two-component developing agent and a one-component developing agent. When the two-component developing agent is used, a bearing particle called a carrier and a toner particle are charged by friction between the carrier and the toner particle, whereby a proper quantity of positive or negative charge is imparted to the toner particle. When the one-component developing agent is used, a toner is used alone as in the case of a magnetic toner. In particular, the two-component developing agent is widely used because the design is easier in the case of the two-component developing agent. This is because the carrier in the two-component developing agent may have functions such as agitating, transporting, and imparting charge, that is, the functions required for a developing agent can be separated and allotted to each of the carrier and the toner.
However, since the two-component developing agent utilizes frictional charging for charging the toner particles, the charge level is changed easily by the influence of environmental change. That is, generally, the charge level tends to be high in a condition of low temperature and low humidity, while the charge level tends to be low in a condition of high temperature and high humidity. Hence, the two-component developing agent has had problems of: reduction in density when the charge level of the toner is made high by a change in the environment; and fog generation when the charge level of the toner is made low by changes in environment.
Manufacturing methods of a toner are classified in a broad sense into a dry method, which uses a conventional melt-kneading pulverization method, and a wet method, in which a toner particle is produced in a solution. The wet method is becoming valued highly from the viewpoints of decreasing the particle size of the toner, narrowing particle size distribution, providing wider freedom of shape control, and reducing energy cost in manufacture. However, since toner particles are formed in a solution in the wet method, a hydrophilic group tends to remain on a surface of a toner particle. And since such a remaining hydrophilic group makes the particles high hygroscopic at a high humidity, charging characteristics tend to be deteriorated. Hence, conventional developing agents containing a toner obtained by means of a wet method has had a fault that charging characteristics are degraded at a high humidity.
Since a spherical toner obtained by the wet method has a larger contact area with a carrier, it takes quite a long time for the toner itself to reach a saturation charge quantity. Therefore, when such a toner is used in an actual apparatus, charge quantities of individual toner particles tend to vary, which has led toward a broader toner charge distribution.
In order to improve characteristics such as toner preservability (blocking resistance), transportability, developing property, transferability, and charging property, the molecular weight, the glass transition temperature, and the melting temperature of a binder resin are controlled. In addition, an organic/inorganic particle called an external additive is added on the surface of the toner particle.
Examples of the inorganic particles include hydrophobic powders represented by hydrophobic silica, silica particles further containing alumina or titania, inorganic particles having a hydrophobicity distribution. However, none of the cited inorganic particles can satisfy both the charge stability with respect to the environmental change and the charge retaining characteristic.
Alternatively, there have been proposed surface treated inorganic compounds used as external additives, such as hydrophobicity-imparted vapor phase titanium oxide; hydrophobicity-imparted anatase type titanium oxide; titania particle which has been surface-treated with a coupling agent; and titanium oxide or alumina whose surface has been subjected to an organic treatment and which has a methanol wettability half value of 55% or more (see Japanese Patent Application Laid-Open (JP-A) Nos. 59-52255, 60-112052, 4-40467 and 8-160659). Such surface-treated inorganic compounds still have a problem that the charge retaining characteristic is not satisfactory, though charge stability with respect to the environmental change is somewhat improved.
Furthermore, there has been proposed a surface-treated titanium oxide particle having a total content of water-soluble components of no higher than 0.2% by mass, with attention given to a core material (see, for example, JP-A No. 6-208241). Such a surface-treated titanium particle also still has the problem that the charge retaining characteristic is not satisfactory, though charge stability with respect to the environmental change is somewhat improved.
Various kinds of resin-coated carriers have been studied mainly from the viewpoint of obtaining good charging characteristics. Regarding the effect of a coating resin on the charging properties of the resin-coated carrier, the change in charge accompanied by a change in the environment (environmental dependency) tends to be large when a resin with a higher charging ability is used. For example, a carrier including polymethyl methacrylate as a coating resin has a higher charging level and a larger environmental dependency than a carrier including polystyrene as a coating resin. That is, in general, a resin material having a group with a higher polarity has not only a higher charge level but also larger environmental dependency. On the other hand, in general, a resin material with a lower polarity has not only a lower charge level but also better environmental dependency. As is explained above, it is difficult for each of a toner and a carrier to have both a desired charge level and charging characteristics with small environmental dependency.
Further, a carrier has to have charge retaining characteristic, which means that a desired charge level of the carrier can be retained for a long period of time.
There are problems that a toner component adheres to a surface of a carrier and fixed thereon and that the coating resin of the carrier peels off owing to stress over time. In order to solve such problems, it is proposed that fluororesin, silicone resin, or the like should be used to reduce the surface energy of the coating resin to protect the carrier surface from contamination and that a strength of a coating resin should be raised to suppress peeling-off or breaking-off of the coating resin. However, because low surface energy material has inferior contact characteristics with a core material, it is very difficult to have both contamination resistance and peeling-off resistance simultaneously.
For solving such a problem, a coated carrier coated with a copolymer of a nitrogen-containing fluorinated alkyl (meth)acrylate and a vinyl based monomer, a copolymer of a fluorinated alkyl (meth)acrylate and a nitrogen-containing vinyl based monomer, or the like is proposed (see, for example, JP-A Nos. 61-80161, 61-80162 and 61-80163). This coated carrier has a relative long lifetime and hard to contaminate with a toner and an external additive. However, since a fluororesin has poor charging ability, when a fluororesin is copolymerized with a nitrogen-containing vinyl based monomer having a polar group with a high charging property or with a methyl methacrylate ester monomer having a polar group with a high charging property, the environmental dependency is degraded.
A carrier has been proposed in which an organic charge controlling agent is incorporated in a polyolefin resin coat or in a surface region thereof (see, for example, JP-A No. 8-160674). This carrier is designed aiming at imparting charge environmental stability to the carrier and making it difficult for a charge controlling agent to be separated from the coating resin. Even with this polyolefin resin, however, a problem arises that contamination by a toner component cannot be sufficiently reduced and good charge retainability cannot be obtained.
A carrier has been proposed in which a quaternary ammonium salt compound having the tendency to positively charging when mixed with iron powder is incorporated in a coating resin layer to thereby control the toner charge stably (see, for example, JP-A No. 2001-22133). Also in the case of such a carrier, a problem has arisen that contamination by a toner component cannot be sufficiently reduced such that good charge retainability cannot be obtained.
As described above, it has been very difficult to simultaneously satisfy high charge level, excellent charge retainability, excellent environmental stability, satisfactory contamination resistance and satisfactory durability, which are required for reliability of carrier.
Accordingly, even when particles added onto a toner and coating resin on a carrier are carefully prepared, it is not easy to obtain both the excellent charge environmental stability and the excellent charge retainability. Further, there has been no effective means to narrow a charge quantity distribution, especially, in a case where a spherical toner is used.